The study of the physiology and pathophysiology of the gastric mucosa will be continued along two lines: 1. An attempt will be made to measure the normal permeability of the gastric mucosal capillaries of the dog's gastric mucosa. A major artery of the mucosa will be cannulated so that it can be mechanically perfused at constant rate. The corresponding vein will be cannulated to permit collection of effluent blood. A bolus injection of radioactively labeled substances will be injected intraarterially, and the venous blood will be collected. Substances injected will include radioactive sodium, tritiated water and two proteins of greatly differing sizes. The permeability and surface area will be calculated from the pattern of the substances in venous blood. 2. The factors affecting permeability of the mucosa's tight junctions will be studied. Among these will be fibrinolysin which is apparently activated within the mucosa during periods of damage.